How to decrease the number of cells that are selected in the formula in Excel?

I'm using the AVERAGE function and drag/expand it along one row (e.g. from A1 to D1), the cells that are selected should decrease by 1 row in each new column.

For example:

A1  =AVERAGE(A2:A10)B1  =AVERAGE(B2:B9)C1  =AVERAGE(C2:C8)

etc.